readahead: improve heuristic detecting sequential reads

Introduce ra.offset and store in it an offset where the previous read
ended.  This way we can detect whether reads are really sequential (and
thus we should not mark the page as accessed repeatedly) or whether they
are random and just happen to be in the same page (and the page should
really be marked accessed again).

Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: WU Fengguang <wfg@mail.ustc.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7c0077f06e24..0949e243b8b9 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -702,6 +702,7 @@ struct file_ra_state {
         unsigned long ra_pages;         /* Maximum readahead window */
         unsigned long mmap_hit;         /* Cache hit stat for mmap accesses */
         unsigned long mmap_miss;        /* Cache miss stat for mmap accesses */
+        unsigned int offset;            /* Offset where last read() ended in a page */
 };
 #define RA_FLAG_MISS 0x01       /* a cache miss occured against this file */
 #define RA_FLAG_INCACHE 0x02    /* file is already in cache */